media: i2c: adv748x: Report correct DV timings for pattern generator

If the pattern generator is enabled the device shall not be queried for
timings. Instead the timings programmed shall be reported as they are
the ones being used to generate the pattern.

Before this change an external HDMI source needed to be connected for
the pattern generator to work. The driver would query this external
HDMI source for timings and program the pattern generator using those.

With this change the user can control the timings and have the pattern
generator work without the need of an external HDMI source being
connected.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>